Display screen or portion thereof with a graphical user interface



The sole FIGURE is a display screen or portion thereof with a graphical user interface showing our new design.

The peripheral dashed broken lines illustrate a display screen or portion thereof, and form no part of the claimed design. The dashed broken lines within the peripheral dashed broken lines show portions of a graphical user interface that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 